2FSK deviation with different filters

Hello!
I did measurements of deviation for 2FSK modulated signal (45 kHz deviation and symbol rate 100k) with two different filter types: Gaussian 0.5 and Rectangular.
Deviation was measured with “Digital modulation analysis”.
During these measurements synchronization via video trigger was used. Measurements were done by the BB60C spectrum analyzer.

Measurements were made with two different generators:
1) Signal Hound VSG60 Vector Generator;
2) Rohde & Schwarz SMIQ 06B signal generator.

For Signal Hound VSG60 Vector Generator, deviation was measured about 44.2 kHz for both types of filters (Gaussian 0.5 and Rectangular).

For Rohde & Schwarz SMIQ 06B signal generator, result of deviation measurement for Gaussian 0.5 was about 44.5 kHz. For Rectangular filter result of deviation measurement is about 41.5 kHz.

Why I have a different results for two generators while modulation settings for both generators were the same?

Regards, Val

Val,

Since the Gaussian filter is not a zero-ISI filter, both the filter length and data itself will affect the deviation. You would have to make sure the lengths match, and then do some sort of sync word synchronization to ensure you were measuring the same exact bit sequence in the software. And even then there might be slight differences in the implementation that lead to freq deltas, but I suspect that would eliminate most of the difference.

If you need ultimate control over the symbols and deviation, you could generate your own I/Q waveform and transmit it with the VSG’s.

Otherwise, what you are seeing seems normal.

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